Eric Lomax, who had been a PoW in Japan during the war

Firth told yesterday how he had been left “overwhelmed” after meeting the man who has inspired The Railway Man and his wife, who will be played by Nicole Kidman, in what is being billed as epic true-life drama.

The two actors brought the country’s latest brush with Hollywood glamour to Princes Street at a press conference to herald the start of a month-long shoot in and around Edinburgh on The Railway Man, which explores the impact of years of brutality and torture as Lomax worked on the “Death Railway” in the Far East.

Firth has twice travelled to Lomax’s home in Berwick-upon-Tweed to meet the couple as part of his research on the film, which will chart more than 70 years of his life.

Jeremy Irvine, who shot to fame in Oscar-winning War Horse, will play the young Lomax, who was brutally treated after being found with a clandestine radio.

Lomax, who was originally from Portobello, was an army signals officer captured in the 1941 fall of Singapore and later sent to the Burmese border jungle prison.

He famously won the UK’s premier award for non-fiction in 1996 – three years after his wife helped organise a meeting at the River Kwai with Nagase Takashi, the Japanese soldier who became his main tormentor, dramatic scenes which will be re-enacted in the film.

It was several decades after the war that Lomax met his wife-to-be on a train journey. Firth said: “They [Eric and Patti] are both incredibly engaging and made me feel very welcome. I found them both a delight, although I did feel at times a little overwhelmed by the enormity of the story.

“He is 92 and not really demonstrating that at all. He is mentally far more agile than I am. I have to keep up with him really. He has a tremendous sense of humour that can be a little dark at times.

“It was important to me to meet them. It focused me and it was something that was very sobering, but also the story is such a big one and about a generation prior to my own, it can feel a bit abstract, a little bit out of reach. But to meet Eric personalised it and humanised it.”

Kidman, who relived a teenage “road trip” around Scotland with a boyfriend yesterday, revealed she will also be meeting the couple during a shoot which will take them into rural locations around North Berwick and Port Seton.

“I wanted to form the character first and then meet her, so I was not trying to force myself into being her,” she said. “I have to find my own way. She is very much the heroine of this film; that is what drew me to the role. I found the subject matter very moving.”

The two actors said they were struck by how much relevance Lomax’s story still held today. Firth said: “Just about any era you could name could be held to be famous for its brutality and devilish tortures. Sadly, I can’t envisage a time when it won’t be relevant.”

Kidman, who said she was “polishing” her English accent for the role, added: “I just found the subject matter very moving – that’s what drew me to it and the power of somebody loving someone through trauma. I found that inspiring.

“The day after I took the part I read an article in the New York Times about a woman who had fallen in love with a man who had just got back from Afghanistan. She had met him and got engaged. It was a very powerful article. I thought: ‘There’s a sign that this is still incredibly relevant.’

Last year, Lomax said of his experiences: “Continuing to hate gets you nowhere. It just damages you as an individual. At some point, the hating has to stop.”

The cast and crew of The Railway Man, who also include veteran Swedish actor Stellan Skarsgard, and one of Japan’s biggest stars, Hiroyuki Sanada, will spend around a month on the east coast of Scotland before location filming moves to Thailand and then Australia. The Australian director of The Railway Man, Jonathan Teplitzky, joked that “the weather” in Scotland had been a key factor in bringing the shoot here.

He added: “We are all captured by the story and Eric’s journey, and I think it’s a story that began here and, to a certain extent, we just wanted to honour that and capture the story of his journey.”

Producer Andy Paterson said part of the reason for wanting to film in Edinburgh was his experience shooting the 1980s film Restless Natives in Scotland.

Paterson, who has harboured ambitions to make The Railway Man for more than a dozen years after reading Lomax’s book, said: “We’ll be using lots of different locations for The Railway Man. We’ll be in Edinburgh, we’ll be using some trains, I think it’s well documented now that we’ll be using a house in North Berwick. We’ll be shooting in Berwick itself… we’ll be dotted about the place.

“It’s a very big production, it’s an emotional thriller. It’s one of those stories that once you’ve read it you never let it go.”

Andrew Dixon, chief executive of Creative Scotland, which is ploughing £300,000 into the production, said he was “very hopeful” that the film would be shown at the Edinburgh International Film Festival next year.